It is well known that pain can be alleviated by electrical pulses applied to the surface of the body or to electrodes implanted within the body. Initially, this electrical stimulation was applied in such a manner that the energy was only sufficient to stimulate superficial sensory nerves, and effort was made to avoid stimulating deep sensory and muscle nerves which produce fasciculations (twitch contractions). Subsequently it was found that electrical stimulation at sufficiently high levels to elicit muscle contractions resulted in greatly improved long term analgesia, with substantial pain relief carryover after ceasing stimulation, but pain patients were in general unable to tolerate the unpleasant sensations which accompanied the high intensity stimulation.
To avoid this it has been proposed to stimulate muscle and deep sensory nerves with short trains of electrical pulses, the pulse trains being at a low frequency. By this procedure, the current can be increased by one-third to one-half obtaining the necessary contraction intensity level without the painful response from the stimulus. Conventional transcutaneous electric nerve stimulation usually consists of a continuous train of pulses with three variable parameters. The rate may vary between 1 to 100 pulses per second, output between 0 to 70 milliampheres peak-to-peak, and pulse width between 0 to 400 micro-seconds. High rate transcutaneous electrical nerve stimulation usually refers to rates greater than 50 pulses per second. At these higher rates, if the intensity is increased to a level which produces muscle contractions, few patients can tolerate the resulting painful stimulus sensations. By interrupting the pulse train periodically at a low rate, that is, by cycling the stimulation on and off, the sensations can be reduced to a degree, but generally patients cannot tolerate even interrupted pulses at the level necessary for muscle stimulation due to adverse reaction to the low rate of interruption of the current often referred to as "Chinese water torture sensation". By "modulating", that is by varying automatically the stimulus parameters in a prescribed manner, one produces a new sensation that is different than any one setting of parameters. The particular manner of automatic variation is important, that is whether rate is varied alone, amplitude alone, pulse width alone, or in various combinations. In addition, it is of vital importance to provide a means for the patient to adjust individual parameters of the stimulus manually during the modulation so as to achieve a base range of parameters that they perceive as more pleasant than others. No single combination of parameters is optimum for the patient, rather it is the variations around base parameters that is important, and the patient must determine this setting while the stimulator is modulated.
In addition, an automatic means must be provided such that the percent of modulation increases with increases in the manual setting of parameters by the patient, as high rates, wide pulses and higher intensity mask the changes caused by modulation, there the change must be larger. Also, at low levels of intensity for a sensitive patient the percent change must be smaller in order to prevent startling or causing painful sensation.